Table 1.

The magnitude of sound pressure signal of different drilling conditions along the whole frequency spectrum (mean ± standard deviation).

Tissues	Frequency (kHz)
	1	2	3	4	5
Annulus fibrosus	19.88±2.31	25.17±6.28	2.64±0.61	5.79±1.5	8.6±3.88
Endplate cartilage	26.19±0.8	21.96±2.82	8.68±0.86	1.43±0.51	5.72±1.82
Sub-endplate cortical bone	72.97±0.59	52.4±2.88	21.3±1.78	20.62±3.36	7.49±1.86
PLL	22.41±1.18	10.04±3.19	1.38±1.37	2.16±0.65	1.25±0.53
Penetration	1.15±0.36	0.75±0.16	0.68±0.15	0.7±0.18	0.61±0.12